One of the primary steps in the fabrication of modern semiconductor devices is the formation of a film, such as a silicon oxide, on a semiconductor substrate. Silicon oxide is widely used as an insulating layer in the manufacture of semiconductor devices. As is well known, a silicon oxide film can be deposited by thermal chemical vapor deposition (CVD) or a plasma chemical vapor deposition processes among other techniques. In a conventional thermal CVD process, reactive gases are supplied to the substrate surface where heat-induced chemical reactions (homogeneous or heterogeneous) take place to produce a desired film. In a conventional plasma process, a controlled plasma is formed to decompose and/or energize reactive species to produce the desired film.
Semiconductor device geometries have dramatically decreased in size since such devices were first introduced several decades ago. Smaller feature sizes have resulted in the presence of increased aspect ratio gaps for some applications, for example, between adjacent conductive lines or in etched trenches. The aspect ratio of a gap is defined by the ratio of the gap""s height or depth to its width. These spaces are difficult to fill using conventional CVD methods. A film""s ability to completely fill such gaps is referred to as the film""s xe2x80x9cgapfillingxe2x80x9d ability. Silicon oxide is one type of insulation film that is commonly used to fill the gaps in intermetal dielectric (IMD) applications, premetal dielectric (PMD) applications and shallow trench isolation (STI) applications among others. Such a silicon oxide film is often referred to as a gapfill film or a gapfill layer.
Some integrated circuit manufacturers have turned to the use of high density plasma CVD (HDP-CVD) systems to deposit silicon oxide gapfill layers. HDP-CVD systems form a plasma that is approximately two orders of magnitude or greater than the density of a standard, capacitively-coupled plasma CVD system. Examples of HDP-CVD systems include inductively-coupled plasma systems and electron cyclotron resonance (ECR) plasma systems among others. HDP-CVD systems generally operate at lower pressure ranges than low density plasma systems. The low chamber pressure employed in HDP-CVD systems provides active species having a long mean-free-path and reduced angular distribution. These factors, in combination with the plasma""s density, contribute to a significant number of constituents from the plasma reaching even the deepest portions of closely spaced gaps, providing a film with improved gapfill capabilities as compared to films deposited in a low density plasma CVD system.
Another factor that allows films deposited by HDP-CVD techniques to have improved gapfill characteristics as compared to films deposited by other CVD techniques is the occurrence of sputtering, promoted by the plasma""s high density, simultaneous with film deposition. The sputtering element of HDP deposition slows deposition on certain features, such as the corners of raised surfaces, thereby contributing to the increased gapfill ability of HDP deposited films. Some HDP-CVD systems introduce argon or a similar heavy inert gas to further promote the sputtering effect. These HDP-CVD systems typically employ an electrode within the substrate support pedestal that enables the creation of an electric field to bias the plasma toward the substrate. The electric field can be applied throughout the HDP deposition process to generate sputtering and provide better gapfill characteristics for a given film.
One HDP-CVD process commonly used to deposit a silicon oxide film forms a plasma from a process gas that includes silane (SiH4), molecular oxygen (O2) and argon (Ar). The industry has found that silicon oxide film deposited according to such an HDP-CVD process are useful for a variety of applications and exhibit improved gapfill characteristics as compared to many other silicon oxide film deposition techniques that do not rely on HDP-CVD technology. Recently, however, engineers have discovered that for some high aspect ratio applications where the width of a gap to be filled is in the range of 0.13 microns or less, the addition of argon to the process gas actually hinders gapfill capabilities. FIGS. 1A-1C, which are simplified cross-sectional views of an HDP-CVD silicon oxide film at different stages of deposition, help illustrate this problem. The HDP-CVD film formed in FIGS. 1A-1C was deposited in a Ultima(trademark) HDP-CVD chamber manufactured by Applied Materials, the assignee of the present application, using the process set forth below in Table 1 which was optimized for gapfill properties.
For the gas flow entries within table 1 that include two numbers, the first number indicates the flow rate of the particular gas through side nozzles of the HDP-CVD apparatus while the second number indicates the flow rate of the gas through a top, centered nozzle. Also, TVO means xe2x80x9cthrottle valve fully openxe2x80x9d which results in chamber pressure being controlled by the quantity of gas flowed into the chamber.
FIGS. 1A-1C, which are simplified cross-sectional views of a silicon oxide film at different stages of deposition, illustrate the potential gapfill limitation that is associated with the process recipe of Table 1 for certain small width gaps having relatively high aspect ratios. It is important to understand that while HDP-CVD silicon oxide deposition techniques generally provide for improved gapfill as compared to other plasma silicon oxide deposition techniques including low density, capacitively coupled plasma CVD techniques, the gapfill issues associated with those techniques become an issue for HDP-CVD techniques in certain aggressive gapfill applications, for example, gaps having a width of 0.10 xcexcm and a 5:1 aspect ratio. The gapfill problem illustrated in FIGS. 1A-1C is not drawn to scale in order to more easily illustrate the problem.
FIG. 1A shows the initial stages of film deposition over a substrate (not shown) having a gap 120 defined by two adjacent features 122, 124 formed over the substrate. As shown in FIG. 1A, the conventional HDP-CVD silicon oxide deposition process results in direct silicon oxide deposition on horizontal surface 126 within gap 120 and horizontal surfaces 128 above features 122, 124. The process also results in indirect deposition (referred to as re-deposition) of silicon oxide on sidewalls 130 due to the recombination of material sputtered from the silicon oxide film as it grows. In certain small-width, highxe2x80x94aspectxe2x80x94ratio applications, the continued growth of the silicon oxide film results in formations 132 on the upper section gap sidewall that grow toward each other at a rate of growth exceeding the rate at which the film grows laterally on lower portions 134 of the sidewall (see FIG. 1B also). The final result of this process is that a void 136 forms as shown in FIG. 1C.
One method that semiconductor manufacturers have developed in order to address this issue is to remove the argon from the process gas altogether. Engineers at Applied Materials were able to develop an optimized SiH4 and O2 HDP-CVD process without argon that was able to adequately fill gaps having an aspect ratio of 5:1 and a width of only 0.15 microns. This SiH4 and O2 process, however, has so far proven to be inadequate at completely filling some even more aggressive gapfill applications.
Accordingly, despite the improvement in gapfill capabilities provided by HDP-CVD systems and the relatively good gapfill characteristics of HDP-CVD silicon oxide films in particular, the development of film deposition techniques that enable the deposition of silicon oxide layers having even further improved gapfill characteristics are desirable. Such improved silicon oxide film deposition are particularly desirable in light of the aggressive gapfill challenges presented by integrated circuit designs employing minimum feature sizes of 0.10 microns and less.
Embodiments of the present invention pertain to a method and apparatus for depositing silicon oxide films having improved gapfill capabilities. Embodiments of the method of the invention deposit such films using HDP-CVD deposition techniques and are particularly useful for premetal dielectric and shallow trench isolation applications in the manufacture of integrated circuits having minimum feature sizes of 0.10 microns or less. As used herein, a high density plasma is a plasma having an ion density of at least 1xc3x971011 ions/cm3.
In one embodiment, the method includes flowing a process gas consisting of a silicon-containing source, an oxygen-containing source and helium into a substrate processing chamber and forming a plasma from the process gas. The inventors have found that adding helium to the process gas instead of argon actually improves gapfill capabilities of the process at such small minimum features sizes. In order to achieve the improved gapfill capabilities, however, the inventors have found that is important to flow the helium source at a rate such that the ratio of the flow rate of helium with respect to the combined flow rate of the silicon source and oxygen source is at least 0.5:1. According to some embodiments of the invention, the ratio of the flow rate of helium with respect to the combined flow rate of the silicon source and oxygen source is between 0.5:1 to 3.0:1 inclusive. In one particular embodiment, the process gas consists of monosilane (SiH4), molecular oxygen (O2) and helium. Also, in some embodiments, the pressure level within the chamber is maintained at or below about 7 mTorr.
According to another embodiment, a method of forming a shallow trench isolation structure in a semiconductor substrate is disclosed. The method includes forming a silicon nitride layer over the substrate and etching a plurality of trenches in the substrate through the silicon nitride layer. Next, a silicon oxide layer is deposited over the substrate to fill the trenches by flowing a process gas consisting of a silicon source, an oxygen-containing source and helium into the substrate processing chamber and forming a high density plasma from said process gas. The ratio of a flow rate of the helium source to a combined flow rate of the silicon-containing source and the oxygen-containing source in the process gas is between 0.5:1 and 3.0:1 inclusive.
These and other embodiments of the present invention, as well as its advantages and features are described in more detail in conjunction with the text below and attached figures.